How a Sun Weather Lull Affects Earth

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The sun’s activity hit a dramatic low in 2008, a historic lull
that caused a similar drop in magnetic effects on Earth — with an
eight-month lag, a new study suggests.

The study found that many magnetic changes on Earth are indeed
strongly linked to the solar
activity cycle, though not in perfect synchrony, and it can
help scientists map out some causes. The speed of the solar wind
— the 1-million-mph stream of particles coming from the sun — as
well as the strength and direction of the magnetic fields
embedded in it helped produce the low readings on our planet,
researchers said.

"Historically, the solar minimum is defined by
sunspot
number," said study lead author Bruce Tsurutani, of NASA's
Jet Propulsion Laboratory in Pasadena, Calif., in a statement.
"Based on that, 2008 was identified as the period of solar
minimum. But the geomagnetic effects on Earth reached their
minimum quite some time later, in 2009. So we decided to look at
what caused the geomagnetic minimum." [ Photos :
Sunspots
on
Earth ' s
Closest
Star ]

Three big factors

The sun typically follows an 11-year cycle, with periods of high
activity known as solar maximums and the lulls classified as
solar minimums. Currently, the sun is in an active phase of its
weather cycle. [ Amazing
New Sun Photos From Space ]

The current solar activity cycle is called Solar Cycle 24. It
looks like Solar Cycle 25 could be an extremely low period,
according to new
research announced today at the annual meeting of the solar
physics division of the American Astronomical Society.

Three things help determine the amount of energy transferred from
the sun to Earth's magnetosphere: the speed of the solar wind,
the strength of the magnetic field outside Earth (known as the
interplanetary magnetic field) and which direction this field is
pointing. The research team looked at each of these factors.

The scientists found that the interplanetary magnetic field was
extraordinarily low in 2008 and 2009. This was an obvious
contribution to the geomagnetic minimum. But it couldn't be the
only explanation, researchers said, since Earth magnetic effects
dropped in 2009 but not 2008.

Using NASA's Advanced Composition Explorer satellite, the team
discovered that the solar wind remained high during the sunspot
minimum in 2008. It began a steady decline later, however —
consistent with the timing of the decline in geomagnetic effects.

The placement of 'coronal holes'

Further investigation revealed the cause of this decrease:
phenomena called coronal holes. Coronal holes are relatively
dark, cold areas within the sun's outer atmosphere. Solar wind
rockets at great speeds from the centers of these holes and much
more slowly from their edges.

During a solar minimum,
coronal holes are usually found at the sun's poles, sending to
Earth only the slow-moving wind from the holes' edges, not the
fast stuff from their centers. But this wasn't the case in 2008,
researchers said. Rather, the holes lingered for a while at low
latitudes before finally migrating to the poles in 2009.

Only then, researchers said, did the speed of the solar wind at
Earth begin to slow down, leading to a decrease in geomagnetic
effects, which can manifest as variably intense auroras — the
brilliant light shows found near Earth's poles.

So researchers are starting to get a handle on what causes
geomagnetic minimums: low interplanetary magnetic field strength,
along with slower solar wind speed and coronal hole placement.

"It's important to understand all of these features better,"
Tsurutani said. "This is all part of the solar cycle, and all
part of what causes effects on Earth."

The study appeared in the May 16 issue of the journal Annales
Geophysicae.

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